Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys, though such alloys alone often do not retain adequate mechanical properties for components located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to thermally insulate such components in order to minimize their service temperatures. For this purpose, thermal barrier coatings (TBC) formed on the exposed surfaces of high temperature components have found wide use.
To be effective, thermal barrier coatings must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between materials having low thermal conductivity and superalloy materials typically used to form turbine engine components. Thermal barrier coating systems capable of satisfying the above requirements have generally required a metallic bond coat deposited on the component surface, followed by an adherent ceramic layer that serves to thermally insulate the component. In order to promote the adhesion of the ceramic layer to the component and inhibit oxidation of the underlying superalloy, the bond coat is typically formed from an oxidation-resistant aluminum-containing alloy such as MCrAlY (where M is iron, cobalt and/or nickel), or by an oxidation-resistant aluminum-based intermetallic such as nickel aluminide or platinum aluminide.
Various ceramic materials have been employed as the ceramic layer, particularly zirconia (ZrO.sub.2) stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO), ceria (CeO.sub.2), scandia (Sc.sub.2 O.sub.3), or another oxide. These particular materials are widely employed in the art because they can be readily deposited by plasma spray, flame spray and vapor deposition techniques, and are reflective to infrared radiation so as to minimize the absorption of radiated heat. In order to increase the resistance of the ceramic layer to spallation when subjected to thermal cycling, the prior art has proposed ceramic layers having enhanced strain tolerance as a result of the presence of porosity, microcracks and segmentation of the ceramic layer. Thermal barrier coating systems employed in higher temperature regions of a gas turbine engine are typically deposited by physical vapor deposition (PVD) techniques that yield a columnar grain structure that is able to expand without causing damaging stresses that lead to spallation.
The bond coat is also critical to promoting the spallation resistance of a thermal barrier coating system. As noted above, bond coats provide an oxidation barrier for the underlying superalloy substrate. Conventional bond coat materials contain aluminum, such as diffusion aluminides and MCrAlY alloys noted above, which enables such bond coats to be oxidized to grow a strong adherent and continuous aluminum oxide layer (alumina scale). The oxide layer chemically bonds the ceramic layer to the bond coat, and protects the bond coat and the underlying substrate from oxidation and hot corrosion.
Though bond coat materials are particularly alloyed to be oxidation-resistant, oxidation inherently occurs over time at elevated temperatures, which gradually depletes aluminum from the bond coat. In addition, aluminum is lost from the bond coat as a result of diffusion into the superalloy substrate. Eventually, the level of aluminum within the bond coat is sufficiently depleted to prevent further growth of aluminum oxide, at which time spallation may occur at the interface between the bond coat and the oxide layer. In addition to depletion of aluminum, the ability of the bond coat to form the desired aluminum oxide layer can be hampered by the diffusion of elements from the superalloy into the bond coat, such as during formation of a diffusion aluminide coating or during high temperature exposure. Oxidation of such elements within the bond coat can become favored as the aluminum within the bond coat is depleted through oxidation and interdiffusion.
From the above, it is apparent that the service life of a thermal barrier coating is dependent on the bond coat used to anchor the thermal insulating ceramic layer, which is prone to degradation over time at elevated temperatures as a result of depletion of aluminum and interdiffusion with the superalloy substrate. Once spallation of the ceramic layer has occurred, the component must be refurbished or scrapped at considerable cost. Therefore, it would be desirable if further improvements were possible for the service life of a thermal barrier coating system.